Centrifugal-chiller and method for controlling the same

ABSTRACT

Provided is a centrifugal-chiller including one variable-speed centrifugal-compressor and one constant-speed centrifugal-compressor and capable of high-efficiency operation. A centrifugal-chiller comprising a variable-speed centrifugal-compressor whose rotational frequency can be varied by an inverter and which compresses a refrigerant; a constant-speed centrifugal-compressor connected in parallel with the variable-speed centrifugal-compressor and operated at a constant rotational speed to compress the refrigerant; and a control unit that controls the operations of the variable-speed centrifugal-compressor and the constant-speed centrifugal-compressor, wherein in the case where a required load factor of the centrifugal-chiller is less than a predetermined value, the control unit selects a variable-speed-compressor priority operating mode in which only the variable-speed centrifugal-compressor is activated, and in the case where the required load factor of the centrifugal-chiller is the predetermined value or greater, the control unit selects a combined-use operating mode in which the variable-speed centrifugal-compressor and the constant-speed centrifugal-compressor are activated.

TECHNICAL FIELD

The present invention relates to a centrifugal-chiller equipped with avariable-speed centrifugal-compressor and a constant-speedcentrifugal-compressor and to a method for controlling the same.

BACKGROUND ART

Known heat source systems for use in supplying chilled water insemiconductor factories and zoned air conditioning are equipped withcentrifugal-chillers. The centrifugal-chillers are equipped withcentrifugal-compressors for compressing refrigerant. In general, onecentrifugal-compressor is provided for one centrifugal-chiller. However,in the case where manufacturing costs can be reduced by using availablegeneral-purpose centrifugal-compressors or to cope with high capacitywhere sufficient performance cannot be achieved by onecentrifugal-compressor, two centrifugal-compressors are sometimesprovided for one centrifugal-chiller. In this case, however,centrifugal-compressors having the same capacity are generally disposedin parallel in consideration of ease of maintenance or operation.

On the other hand, for package air conditioners based on a principledifferent from that of the centrifugal-chillers, a known approach is todispose two compressors in parallel. These compressors are a combinationof a variable-speed compressor whose rotational frequency can be variedusing an inverter and a constant-speed compressor operated at a constantrotational speed.

CITATION LIST Patent Literature

-   {PTL 1} Japanese Unexamined Patent Application, Publication No. Hei    7-35425-   {PTL 2} Japanese Unexamined Patent Application, Publication No. Hei    7-83523

SUMMARY OF INVENTION Technical Problem

However, the compressors described in the foregoing Patent Literaturesassume a scroll compressor or a rotary compressor and fundamentallydiffer in the principle of operation from a centrifugal-compressor,which is an aerodynamic rotary machine. Thus, an approach completelydifferent from the scroll compressor and the rotary compressor isrequired in consideration of enhancing the efficiency ofcentrifugal-chillers. For example, a centrifugal-chiller that uses avariable-speed centrifugal-compressor whose rotational frequency can bevaried using an inverter shows a high COP (coefficient of performance ofthe centrifugal-chiller) in regions at a low cooling-water temperature(outside-air wet-bulb temperature) and shows an extremely high COP inregions at a low cooling-water temperature and a low compressor loadfactor. Furthermore, it has the characteristic that the COP is decreasedby an amount corresponding to an inverter loss at a high cooling-watertemperature and a high compressor load factor as compared with theconstant-speed compressor operated at a constant rotational speed. Onthe other hand, a centrifugal-chiller that uses a constant-speedcentrifugal-compressor shows a low COP at a low compressor load factorand shows a high COP at a high compressor load factor. Thus, with theuse of two compressors, that is, one variable-speedcentrifugal-compressor and one constant-speed centrifugal-compressor,for one centrifugal-chiller energy-saving combinations and operationcontrol methods can be expected.

Furthermore, in the case where the capacity of the centrifugal-chilleris so large that two centrifugal-compressors are needed, twoconstant-speed centrifugal-compressors may be provided; however,enhancement of the efficiency cannot be expected at a low compressorload factor. On the other hand, two variable-speedcentrifugal-compressors may be provided, in which case two inverters areneeded, thus posing problems in terms of cost and maintenance. Thus, acombination of one variable-speed centrifugal-compressor and oneconstant-speed centrifugal-compressor may be provided. At present,however, there are no proposals to optimally enhance the efficiency ofsuch a combination of centrifugal-compressors.

The present invention is made in consideration of such circumstances andprovides a centrifugal-chiller equipped with one variable-speedcentrifugal-compressor and one constant-speed centrifugal-compressor toallow a high efficiency operation, as well as a method for controllingthe same.

Solution to Problem

To solve the above problems, a centrifugal-chiller and a method forcontrolling the same of the present invention adopt the followingsolutions.

A centrifugal-chiller according to a first aspect of the presentinvention includes a variable-speed centrifugal-compressor whoserotational frequency can be varied by an inverter and which compresses arefrigerant; a constant-speed centrifugal-compressor connected inparallel with the variable-speed centrifugal-compressor and operated ata constant rotational speed to compress the refrigerant; a condenserthat condenses the refrigerant compressed by the variable-speedcentrifugal-compressor and/or the constant-speed centrifugal-compressorinto a liquid using cooling water supplied thereto; an expansion valvethat expands the refrigerant that is condensed into a liquid by thecondenser; an evaporator that evaporates the refrigerant expanded by theexpansion valve; and a control unit that controls the operations of thevariable-speed centrifugal-compressor and the constant-speedcentrifugal-compressor, wherein in the case where a required load factorof the centrifugal-chiller is less than a predetermined value, thecontrol unit selects a variable-speed-compressor priority operating modein which only the variable-speed centrifugal-compressor is activated,and in the case where the required load factor of thecentrifugal-chiller is the predetermined value or greater, the controlunit selects a combined-use operating mode in which the variable-speedcentrifugal-compressor and the constant-speed centrifugal-compressor areactivated.

A centrifugal-chiller that uses a variable-speed centrifugal-compressorwhose rotational frequency can be varied by an inverter has thecharacteristic of showing a relatively high COP (coefficient ofperformance of the centrifugal-chiller) at a low compressor load factor,and showing a decreased COP at a high compressor load factor. Thistendency is notable at a low cooling water temperature as in the winterseason.

On the other hand, a centrifugal-chiller that uses a constant-speedcentrifugal-compressor operated at a constant rotational speed shows alow COP at a low compressor load factor and shows a high COP at a highcompressor load factor. This tendency is notable at a high cooling watertemperature as in the summer season.

In the centrifugal-chiller according to the first aspect of the presentinvention, in the case where the load factor of the centrifugal-chilleris less than a predetermined value, the variable-speed-compressorpriority operating mode in which only the variable-speedcentrifugal-compressor whose compressor load factor is less than thepredetermined value is selected, and in the case where the load factorof the centrifugal-chiller is the predetermined value or greater, thecombined-use operating mode in which both the variable-speed and theconstant-speed centrifugal-compressors are activated is selected becausethe compressor load factors of the individual compressors are thepredetermined value or greater in consideration of the characteristics.In the case of operation of only one compressor, the centrifugal-chillerload factor corresponds to the load factor of the compressor, and in thecase of operation of two compressors, it corresponds to the sum of theload factors of the compressors.

In general, in a state in which the cooling water temperature isrelatively low as in the winter season or in the intermediate seasons,the centrifugal-chiller load factor frequently becomes low and thecompressor load factor becomes less than a predetermined value, in whichcase, a high-efficiency operation is achieved only by the variable-speedcentrifugal-compressor by selecting the variable-speed-compressorpriority operating mode. On the other hand, since thecentrifugal-chiller load factor rises, so that the compressor loadfactor becomes the predetermined value or greater in the summer seasonduring which the cooling water temperature is relatively high, ahigh-efficiency operation can be achieved by shifting to thecombined-use operating mode. Thus, a centrifugal-chiller that shows ahigh COP across wide compressor load factors can be achieved bycombining the variable-speed-compressor priority operating mode and thecombined-use operating mode.

In the centrifugal-chiller according to the first aspect of the presentinvention described above, the predetermined value of the load factor ofthe centrifugal-chiller may be set to 30% or greater and less than 50%.

The predetermined value of the load factor of the centrifugal-chiller,that is, a threshold for switching between the variable-speed-compressorpriority operating mode and the combined-use operating mode, is set to30% or greater and less than 50%. This allows the variable-speedcentrifugal-compressor to be operated mainly in thevariable-speed-compressor priority operating mode in which the loadfactor of the centrifugal-chiller is set low, thus allowing thevariable-speed centrifugal-compressor having a relatively low capacityto be selected. That is, it suffices that the variable-speedcentrifugal-compressor have a capacity corresponding to acentrifugal-chiller load factor of 30% or higher and less than 50%.Accordingly, it suffices to employ a variable-speedcentrifugal-compressor having a capacity of about half or less than thatof the constant-speed centrifugal-compressor, which allows ageneral-purpose inverter to be used, thus allowing thecentrifugal-compressor to be configured at low cost.

The predetermined value of the compressor load factor, which serves as athreshold, is preferably set to the lower limit of a region in which theefficiency of the constant-speed centrifugal-compressor is apredetermined value or greater (for example, within 20% of the highestefficiency). This can prevent the constant-speed centrifugal-compressorfrom being activated at a low compressor load factor which woulddecrease the efficiency of the centrifugal-chiller.

The capacity of the constant-speed centrifugal-compressor is selected sothat the sum of it and the capacity of the variable-speedcentrifugal-compressor satisfies the centrifugal-chiller load factor of100%. For example, in the case where the variable-speedcentrifugal-compressor has a capacity corresponding tocentrifugal-centrifugal-chiller load factor of 30%, the constant-speedcentrifugal-compressor is set to a capacity corresponding to acentrifugal-centrifugal-chiller load factor of 70%.

In the centrifugal-chiller according to the first aspect of the presentinvention described above, an inlet guide vane that controls the flowrate of the refrigerant flowing to an impeller of the constant-speedcentrifugal-compressor may be provided upstream of the impeller in theflow of the refrigerant; and a wall facing the inlet guide vane may havea sealing portion that minimizes leakage of the refrigerant through thegap between the inlet guide vane and the wall when the inlet guide vaneis fully closed.

The constant-speed centrifugal-compressor is not activated when thevariable-speed-compressor priority operating mode is selected. Since theconstant-speed centrifugal-compressor is connected in parallel with thevariable-speed centrifugal-compressor, a pressure difference inrefrigerant is generated in front of and behind the constant-speedcentrifugal-compressor even in the variable-speed-compressor priorityoperating mode. When the refrigerant leaks from the high pressure sideto the low pressure side due to the pressure difference, a refrigerantflow that does not contribute to refrigeration is generated, and thus,the performance of the chiller deteriorates.

Thus, the centrifugal-chiller according to the first aspect of thepresent invention has a sealing section that minimizes refrigerantleakage between the inlet guide vane and the wall, thereby minimizingrefrigerant leakage when the inlet guide vane is fully closed. Thus,even in the variable-speed-compressor priority operating mode, theamount of refrigerant leaked via the constant-speedcentrifugal-compressor is minimized, so that deterioration in theperformance of the chiller can be prevented. Furthermore, since therefrigerant leakage in the constant-speed centrifugal-compressor isminimized, there is no need to provide additional automatic on/off valvein the refrigerant piping.

An example of the sealing section is a labyrinth seal formed at the wallside.

A method for controlling a centrifugal-chiller according to a secondaspect of the present invention is a method for controlling acentrifugal-chiller including a variable-speed centrifugal-compressorwhose rotational frequency can be varied by an inverter and whichcompresses a refrigerant; a constant-speed centrifugal-compressorconnected in parallel with the variable-speed centrifugal-compressor andoperated at a constant rotational speed to compress the refrigerant; acondenser that condenses the refrigerant compressed by thevariable-speed centrifugal-compressor and/or the constant-speedcentrifugal-compressor into a liquid using cooling water suppliedthereto; an expansion valve that expands the refrigerant that iscondensed into a liquid by the condenser; and an evaporator thatevaporates the refrigerant expanded by the expansion valve, the methodcontrolling the operations of the variable-speed centrifugal-compressorand the constant-speed centrifugal-compressor, wherein in the case wherea required load factor of the centrifugal-chiller is less than apredetermined value, the control unit selects avariable-speed-compressor priority operating mode in which only thevariable-speed centrifugal-compressor is activated, and in the casewhere the required load factor of the centrifugal-chiller is thepredetermined value or greater, the control unit selects a combined-useoperating mode in which the variable-speed centrifugal-compressor andthe constant-speed centrifugal-compressor are activated.

A centrifugal-chiller that uses a variable-speed centrifugal-compressorwhose rotational frequency can be varied by an inverter has thecharacteristic of showing a relatively high COP (coefficient ofperformance of the centrifugal-chiller) at a low compressor load factor,and showing a decreased COP at a high compressor load factor. Thistendency is notable at a low cooling water temperature as in the winterseason.

On the other hand, a centrifugal-chiller that uses a constant-speedcentrifugal-compressor operated at a constant rotational speed shows alow COP at a low compressor load factor and shows a high COP at a highcompressor load factor. This tendency is notable at a high cooling watertemperature as in the summer season.

In the method for controlling the centrifugal-chiller according to thesecond aspect of the present invention, in the case where the loadfactor of the centrifugal-chiller is less than a predetermined value,the variable-speed-compressor priority operating mode in which only thevariable-speed centrifugal-compressor whose compressor load factor isless than the predetermined value is selected, and in the case where theload factor of the centrifugal-chiller is the predetermined value orgreater, the combined-use operating mode in which both thevariable-speed and the constant-speed centrifugal-compressors areactivated is selected because the compressor load factors of theindividual compressors are the predetermined value or more inconsideration of the characteristics. The centrifugal-chiller loadfactor corresponds to a compressor load factor in the case of operationof only one compressor and corresponds to the sum of compressor loadfactors in the case of the operation of two compressors.

In general, in a state in which the cooling water temperature isrelatively low as in the winter season or in the intermediate seasons,the centrifugal-chiller load factor frequently becomes low and thecompressor load factor becomes less than a predetermined value, in whichcase, a high-efficiency operation is achieved only by the variable-speedcentrifugal-compressor by selecting the variable-speed-compressorpriority operating mode. On the other hand, since thecentrifugal-chiller load factor rises, so that the compressor loadfactor becomes the predetermined value or greater in the summer seasonduring which the cooling water temperature is relatively high, ahigh-efficiency operation can be achieved by shifting to thecombined-use operating mode. Thus, a centrifugal-chiller that shows ahigh COP across wide compressor load factors can be achieved bycombining the variable-speed-compressor priority operating mode and thecombined-use operating mode.

Advantageous Effects of Invention

The centrifugal-chiller and the method for controlling the sameaccording to the present invention provide the following operationaladvantages.

In the case where the load factor of the centrifugal-chiller is lessthan a predetermined value, a variable-speed-compressor priorityoperating mode in which only the variable-speed centrifugal-compressoris activated is selected, and in the case where the load factor of thecentrifugal-chiller is the predetermined value or greater, acombined-use operating mode in which the variable-speedcentrifugal-compressor and the constant-speed centrifugal-compressor areactivated is selected, and thus, a centrifugal-chiller that shows a highCOP across wide compressor load factors can be achieved.

When the cooling water temperature is low as in the winter season duringwhich the cooling water temperature is a predetermined value or less,the variable-speed-compressor priority operating mode is selected, andthus, higher efficiency is achieved.

When the cooling water temperature is high as in the summer seasonduring which the cooling water temperature is a predetermined value orgreater, the combined-use operating mode is selected, and thus, higherefficiency is achieved.

For the intermediate seasons during which the cooling water temperatureis not high as in the summer season and not low as in the winter season,the variable-speed-compressor priority operating mode is selected in thecase where the load factor of the centrifugal-chiller is less than apredetermined value, and the combined-use operating mode is selected inthe case where the load factor of the centrifugal-chiller is thepredetermined value or greater, and a further increased efficiency isachieved.

A sealing section that minimizes refrigerant leakage is provided betweenthe inlet guide vane and the wall, thereby minimizing refrigerantleakage when the inlet guide vane is fully closed. Thus, even in thevariable-speed-compressor priority operating mode, the amount ofrefrigerant leaked via the constant-speed centrifugal-compressor isminimized, so that deterioration in the performance of the chiller canbe prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic configuration diagram showing a heat source systemequipped with centrifugal-chillers according to an embodiment of thepresent invention.

FIG. 2 is a graph showing the COP of the centrifugal-chiller versus theload factor of the chiller unit (centrifugal-chiller).

FIG. 3 is a front view of an inlet guide vane in a fully closed state.

FIG. 4A is a cross-sectional view taken along section line A-A in FIG.3.

FIG. 4B is a cross-sectional view taken along section line B-B in FIG.3.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described hereinbelowwith reference to the drawings.

FIG. 1 shows an embodiment of a heat source system equipped withcentrifugal-chillers according to the present invention. A heat sourcesystem 1 is equipped with a plurality of (two in this embodiment)centrifugal-chillers 3 provided in parallel and a plurality of (four inthis embodiment) cooling towers 5 provided in parallel.

The centrifugal-chillers 3 are each equipped with twocentrifugal-compressors 7 and 8 that compress a refrigerant, a condenser9 that condenses the refrigerant compressed by thecentrifugal-compressors 7 and 8 to liquefy it, an expansion valve (notshown) that expands the refrigerant condensed and liquefied by thecondenser 9, and an evaporator 11 that evaporates the refrigerantexpanded by the expansion valve.

One centrifugal-compressor 7 of the two is driven by an electric motor13 whose rotational frequency can be varied by an inverter. Thus, thecentrifugal-compressor 7 is a variable-speed centrifugal-compressor.

The other centrifugal-compressor 8 is a constant-speedcentrifugal-compressor that is operated at a fixed rotational speed byan electric motor 14.

Cooling water supplied by a cooling-water pump 15 is guided to thecondenser 9. The cooling-water pump 15 is driven by an electric motor(not shown) whose rotational frequency can be varied by an inverter. Theinlet temperature of the cooling water supplied to the condenser 9 andthe output temperature of the cooling water flowing out from thecondenser 9 are measured by temperature sensors (not shown), and theoutput values are transmitted to a control unit, to be described later.

The individual cooling-water pumps 15 suck cooling water guided from acooling-water return header 17 and discharge it toward the condenser 9.The cooling water discharged from the condenser 9 side is guided to acooling-water supply header 19. All of the centrifugal-chillers 3 andall of the cooling towers 5 are connected in common to the cooling-waterreturn header 17. All of the centrifugal-chillers 3 and all of thecooling towers 5 are connected in common also to the cooling-watersupply header 19.

Cooling water supplied by a chilled water pump 21 is guided to theevaporator 11. The chilled-water pump 21 is driven by an electric motor(not shown) whose rotational frequency can be varied by an inverter. Theinlet temperature of the chilled water supplied to the evaporator 11 andthe output temperature of the chilled water flowing out from theevaporator 11 are measured by temperature sensors (not shown), and theoutput values are transmitted to the control unit, to be describedlater. The flow rate of the chilled water is also measured by aflowmeter, and the output value is transmitted to the control unit, tobe described later. The control unit calculates a load consumed by anexternal load by multiplying the difference between the chilled-waterinlet and outlet temperatures, the cooling-water flow rate, the specificheat of the chilled water, and the specific gravity of the chilled watertogether.

The individual chilled water pumps 21 suck chilled water guided from achilled-water return header 23 and discharge it toward the evaporator11. The chilled water discharged from the evaporator 11 side is guidedto a chilled-water supply header 25. All of the centrifugal-chillers 3are connected in common to the chilled-water return header 23. All ofthe centrifugal-chillers 3 are connected in common also to thechilled-water supply header 25.

The chilled-water return header 23 and the chilled-water supply header25 are connected to the external load (not shown). The external load issupplied with chilled water (for example, at 7° C.) cooled by theevaporator 11 through the chilled-water supply header 25, and thechilled water that is used by the external load and has its temperatureincreased (for example, to 12° C.) is returned to the evaporator 11 sidethrough the chilled-water return header 23.

The cooling towers 5 are each provided with a cooling tower fan 30, awater spray header 32, and a cooling-water reservoir tank 34.

The cooling tower fan 30 is used to introduce outside air into thecooling tower 5 and is driven by an electric motor 36. The electricmotor 36 preferably has a rotational frequency that can be varied by aninverter.

The water spray header 32 sprays cooling water into outside air fromabove to bring it into contact with the outside air, thereby cooling thecooling air using not only sensible heat but also latent heat ofvaporization. A cooling-water supply on/off valve 38 is provided betweenthe water spray header 32 and the cooling-water supply header 19.

The cooling-water reservoir tank 34 reserves the cooled cooling watersprayed and cooled by the outside air. The cooling water reserved in thecooling-water reservoir tank 34 is guided to the cooling-water returnheader 17 via a cooling-water return on/off valve 40.

The cooling towers 5 are each activated and stopped by opening andclosing the cooling-water supply on/off valve 38 and the cooling-waterreturn on/off valve 40. Thus, the number of cooling towers 5 to beactivated can be changed.

The cooling towers 5 are each provided with a humidity sensor (notshown). The humidity sensor measures an outside-air wet-bulbtemperature. The output of the humidity sensor is sent to the controlunit, to be described later. The outside-air wet-bulb temperature may bedetermined from a dry-bulb temperature, a relative humidity, and anoutside air pressure, instead of using the humidity sensor.

The heat source system is equipped with the control unit (not shown),and the control unit controls the operations of thecentrifugal-compressors 7 and 8, the expansion valves, the cooling-waterpumps 15, the cooling tower fans 30, the cooling-water supply on/offvalves 38, the cooling-water return on/off valves 40, and the chilledwater pumps 21.

The above-configured heat source system 1 operates as follows.

The centrifugal-compressors 7 and 8 are activated in accordance with aninstruction from the control unit to compress the refrigerant. The orderof activation of the centrifugal-chillers 7 and 8 will be describedlater.

The refrigerant compressed by the centrifugal-chillers 7 and 8 is guidedto the condenser 9 and cooled by cooling water supplied from thecooling-water return header 17 by the cooling-water pump 15 to becondensed to a liquid. The cooling water that has been given latent heatof condensation by the condenser 9 to raise its temperature is guided tothe cooling-water supply header 19 and is guided to the water sprayheader 32 via the cooling-water supply on/off valve 38. The coolingwater is sprayed into the outside air from the water spray header 32 andis cooled by coming into contact with the outside air guided by thecooling tower fan 30. The cooled cooling water is temporarily reservedin the cooling-water reservoir tank 34 and is then guided to thecooling-water return header 17 via the cooling-water return on/off valve40. Thus, the cooling water circulates between the condenser 9 and thecooling tower 5

The liquid refrigerant condensed into a liquid by the condenser 9 isexpanded by the expansion valve (not shown) and is then guided to theevaporator 11. The evaporator 11 evaporates the liquid refrigerant toabsorb latent heat of vaporization from the chilled water to cool thechilled water. The chilled water is guided from the chilled-water returnheader 23 to the evaporator 11 by the chilled water pump 21, is cooledto a desired temperature (for example, 7° C.) by the evaporator 11, andis guided to the chilled-water supply header 25. The chilled-watersupply header 25 is connected to the external load (not shown), wherethe chilled water is used by the external load and has its temperatureincreased to a predetermined temperature (for example, 12° C.) and isthen returned to the chilled-water return header 23.

The refrigerant evaporated by the evaporator 11 is guided to the intakeports of the centrifugal-compressors 7 and 8, where it is compressedagain.

Next, a method for activating the centrifugal-compressors 7 and 8 willbe described using FIG. 2. The horizontal axis in the drawing indicatesthe load factor of the centrifugal-chiller, and the vertical axisindicates the COP of the centrifugal-chiller 3. The load factor of thecentrifugal-chiller is the quotient of the output of thecentrifugal-chiller 3 (the sum of the outputs of thecentrifugal-compressors 7 and 8 in operation), which is required tocontinue an operation necessary to maintain a required chilled-wateroutlet temperature, divided by the sum of the rated outputs of thecentrifugal-chiller 3 (the sum of the rated outputs of thecentrifugal-compressors 7 and 8). The control unit tracks the presentoperating states of the heat source system 1 and the centrifugal-chiller3 and constantly obtains the load factor of the centrifugal-chiller.

In the drawing, broken lines indicate the variable-speedcentrifugal-compressor 7, and two-dot chain lines indicate theconstant-speed centrifugal-compressor 8. The curved lines indicate COPsat cooling-water inlet temperatures (the temperatures of cooling waterflowing into the condenser 9), specifically, 12° C., 20° C., and 32° C.The cooling-water inlet temperature, 12° C., represents the winterseason, 20° C. represents daytime in intermediate seasons and night inthe summer season (hereinafter simply referred to as “intermediateseasons”), and 32° C. represents daytime in the summer season(hereinafter simply referred to as “summer season”).

As shown in the drawing, the centrifugal-chiller that uses thecentrifugal-compressor 7 has the characteristic of showing a relativelyhigh COP at a low centrifugal-chiller load factor and showing adecreased COP at a high centrifugal-chiller load factor, at a lowcooling water temperature (for example, 12° C.), as in the winterseason. On the other hand, the centrifugal-chiller that uses theconstant-speed centrifugal-compressor 8 shows a significantly decreasedCOP at a low centrifugal-chiller load factor and a constant high COP ata high centrifugal-chiller load factor, at a high cooling watertemperature (for example, 30° C.).

This embodiment performs operation control as follows on the basis ofthe characteristics of the centrifugal-compressors described above.

In the case where the load factor of the centrifugal-chiller is lessthan a predetermined value (for example, 30%), only the variable-speedcentrifugal-compressor 7 is activated (variable-speed-compressorpriority operating mode), and in the case where the load factor of thecentrifugal-chiller is the predetermined value or greater, both thecentrifugal-compressors 7 and 8 are activated to the rating(combined-use operating mode). The threshold load factor of thecentrifugal-chiller is not limited to 30% and is suitably set dependingon the characteristics of the centrifugal-chiller. However, it ispreferable to set the threshold value between 30% or more and less than50%.

In the case where the load factor of the centrifugal-chiller, whichserves as the threshold for switching between the operations of thecentrifugal-compressors, is set to 30%, a capacity of about 30% of therated output is selected for the variable-speed centrifugal-compressor7, and a capacity of about 70% of the rating is selected for theconstant-speed centrifugal-compressor 8.

In the region where the centrifugal-chiller load factor is 30% or less,COPs of the variable-speed centrifugal-compressor 7 higher than the COPsof the constant-speed centrifugal-compressor 8 are selected, asindicated by the bold solid lines in the drawing. In general, acentrifugal-chiller load factor of less than 30% is for operation in thewinter season (region A) and the intermediate seasons (region B).Accordingly, a centrifugal-chiller load factor of less than 30% allowshigh-efficiency operation to be achieved by selecting the variable-speedcentrifugal-compressor 7.

In the region where the centrifugal-chiller load factor is 30% or higherand 100% or less, the intermediate values of the COPs of theconstant-speed centrifugal-compressor 8 and the variable-speedcentrifugal-compressor 7 are selected as indicated by the bold solidlines in the drawing. The region where the centrifugal-chiller loadfactor is 30% or more and 100% or less is for operation in theintermediate seasons (region B) and the summer season (region C). Sincethe COP of the constant-speed centrifugal-compressor 8 is sufficientlyimproved at cooling-water inlet temperatures in the intermediate seasonsand the summer season when the load factor of the centrifugal-chillerexceeds 30%, high-efficiency operation can be achieved even bycombined-use operation of the centrifugal-chillers 7 and 8.

Thus, this embodiment can achieve the centrifugal-chiller 3 that showshigh COPs across wide load factors of the centrifugal-chiller bycombining the variable-speed-compressor priority operating mode and thecombined-use operating mode.

Furthermore, the load factor of the centrifugal-chiller, which serves asa threshold for switching between the variable-speed-compressor priorityoperating mode and the combined-use operating mode, is set to 30% orhigher and less than 50%. This allows the variable-speedcentrifugal-compressor 7 to be operated mainly in thevariable-speed-compressor priority operating mode in which the loadfactor of the centrifugal-chiller is set low, thus allowing thevariable-speed centrifugal-compressor 7 having a relatively low capacityto be selected. That is, it suffices that the variable-speedcentrifugal-compressor 7 have a capacity corresponding to acentrifugal-chiller load factor of 30% or more and less than 50%.Accordingly, it suffices to employ a variable-speedcentrifugal-compressor having a capacity of about half or less comparedwith that of the constant-speed centrifugal-compressor 8, which allows ageneral-purpose inverter to be used, thus allowing thecentrifugal-compressor to be configured at low cost.

Next, the configuration of an inlet guide vane provided in theconstant-speed centrifugal-compressor 8 will be described using FIG. 3and FIGS. 4A and 4B.

An inlet guide vane 50 is provided upstream of an impeller (not shown)of the constant-speed centrifugal-compressor 8 and controls the flowrate of the refrigerant flowing to the impeller. The inlet guide vane 50is provided between a central shaft 52 and a peripheral ring 54, inwhich a plurality of vanes 56 are disposed in a radiating pattern. Thevanes 56 are plate-like members each having a substantially sectorshape. The individual vanes 56 rotate about a rotation axis 58 passingthrough the center of the central shaft 52. The state in FIG. 3 shows aclosed state, in which the individual vanes 56 are located so that thesurfaces intersect the channel of the refrigerant. In an open state, theindividual vanes 56 rotate at 90° to positions at which the surfacesthereof are located along the flow of the refrigerant.

FIG. 4A shows a cross section taken along section line A-A in FIG. 3. Asshown in the drawing, a plurality of grooves 54 a are formed in therefrigerant flowing direction (vertically in the drawing) in the innerperipheral surface of the peripheral ring 54 to form a labyrinth seal(sealing portion). Furthermore, as shown in FIG. 4B showing a crosssection taken along section line B-B in FIG. 3, the outer peripheralsurface of the central shaft 52 also has a plurality of grooves 52 a toform a labyrinth seal (sealing portion).

The labyrinth seals are provided between the individual vanes 56 and theperipheral ring 54 and the central shaft 52 in this manner to minimizerefrigerant leakage in the fully closed state. Thus, even in the casewere a pressure difference in refrigerant is generated in front andbehind the constant-speed centrifugal-compressor 8 due to thevariable-speed centrifugal-compressor 7 disposed in parallel even thoughthe constant-speed centrifugal-compressor 8 is not activated, as in thevariable-speed-compressor priority operating mode, the inlet guide vaneis fully closed to minimize refrigerant leakage. Thus, even in thevariable-speed-compressor priority operating mode, the amount ofrefrigerant leaked via the constant-speed centrifugal-compressor isminimized, so that deterioration in the performance of the chiller canbe prevented. Furthermore, since the refrigerant leakage in theconstant-speed centrifugal-compressor 8 is minimized, there is no needto provide an additional on/off valve in refrigerant piping.

REFERENCE SIGNS LIST

-   1 heat source system-   3 centrifugal-chiller-   5 cooling tower-   7 centrifugal-compressor-   9 condenser-   11 evaporator-   13 electric motor-   15 cooling-water pump-   17 cooling-water return header-   19 cooling-water supply header-   21 chilled water pump-   23 chilled-water return header-   25 chilled-water supply header-   30 cooling tower fan-   32 water spray header-   34 cooling-water reservoir tank-   36 electric motor-   38 cooling-water supply on/off valve-   40 cooling-water return on/off valve

1. A centrifugal-chiller comprising: a variable-speedcentrifugal-compressor whose rotational frequency can be varied by aninverter and which compresses a refrigerant; a constant-speedcentrifugal-compressor connected in parallel with the variable-speedcentrifugal-compressor and operated at a constant rotational speed tocompress the refrigerant; a condenser that condenses the refrigerantcompressed by the variable-speed centrifugal-compressor and/or theconstant-speed centrifugal-compressor into a liquid using cooling watersupplied thereto; an expansion valve that expands the refrigerant thatis condensed into a liquid by the condenser; an evaporator thatevaporates the refrigerant expanded by the expansion valve; and acontrol unit that controls the operations of the variable-speedcentrifugal-compressor and the constant-speed centrifugal-compressor,wherein, in the case where a required load factor of thecentrifugal-chiller is less than a predetermined value, the control unitselects a variable-speed-compressor priority operating mode in whichonly the variable-speed centrifugal-compressor is activated, and in thecase where the required load factor of the centrifugal-chiller is thepredetermined value or greater, the control unit selects a combined-useoperating mode in which the variable-speed centrifugal-compressor andthe constant-speed centrifugal-compressor are activated.
 2. Thecentrifugal-chiller according to claim 1, wherein the predeterminedvalue of the load factor of the centrifugal-chiller is set to 30% orhigher and less than 50%.
 3. The centrifugal-chiller according to claim1, wherein an inlet guide vane that controls the flow rate of therefrigerant flowing to an impeller of the constant-speedcentrifugal-compressor is provided upstream of the impeller in the flowof the refrigerant; and a wall facing the inlet guide vane has a sealingportion that minimizes leakage of the refrigerant through the gapbetween the inlet guide vane and the wall when the inlet guide vane isfully closed.
 4. A method for controlling a centrifugal-chillerincluding a variable-speed centrifugal-compressor whose rotationalfrequency can be varied by an inverter and which compresses arefrigerant; a constant-speed centrifugal-compressor connected inparallel with the variable-speed centrifugal-compressor and operated ata constant rotational speed to compress the refrigerant; a condenserthat condenses the refrigerant compressed by the variable-speedcentrifugal-compressor and/or the constant-speed centrifugal-compressorinto a liquid using cooling water supplied thereto; an expansion valvethat expands the refrigerant that is condensed into a liquid by thecondenser; and an evaporator that evaporates the refrigerant expanded bythe expansion valve, the method controlling the operations of thevariable-speed centrifugal-compressor and the constant-speedcentrifugal-compressor, wherein in the case where a required load factorof the centrifugal-chiller is less than a predetermined value, thecontrol unit selects a variable-speed-compressor priority operating modein which only the variable-speed centrifugal-compressor is activated,and in the case where the required load factor of thecentrifugal-chiller is the predetermined value or greater, the controlunit selects a combined-use operating mode in which the variable-speedcentrifugal-compressor and the constant-speed centrifugal-compressor areactivated.